1. Field of the Invention
This invention relates to a reset circuit for microcomputers and more particularly, to a reset circuit for microcomputers installed in electric apparatuses.
2. Description of the Prior Art
Microcomputers are commonly installed in various electric apparatuses as controller units for controlling the operation thereof. Such a built-in microcomputer as installed in an electric apparatus implements various operational steps in accordance with a contained control program upon receipt of an external operator command or a signal detected by a sensor and the like. It is common to provide such electric apparatus with an auto-reset circuit for resetting the built-in microcomputer simultaneously with plugging an attachment plug of the apparatus in a receptacle in order to initiate the implementation of operational steps by the microcomputer from an initial step of the program.
An example of conventional auto-reset circuit is shown in FIG. 1, wherein the microcomputer is represented by a Central Processing Unit (CPU). Other peripheral devices, such as a RAM, ROM, I/O and the like are not shown so as to reduce circuit complexity.
When a power is supplied to the apparatus by plugging the attachment plug in the receptacle, operating voltages V.sub.c1 and V.sub.c2 are applied to the CPU 1 and auto-reset circuit, respectively. The operating voltage V.sub.c1 charges a capacitor 202, which is used for a back-up voltage source, through a diode 201 and supplies an electric power to a collector of a transistor 204 through a resistor 203.
On the other hand, the voltage V.sub.c2 is fed to a base of the transistor 204 through a differentiating circuit consisting of a resistor 205 and a capacitor 206. Accordingly, the transistor 204 is brought into conduction by a base current (or a charging current to the capacitor) that flows through the resistor 205 and the capacitor 206, and then a collector voltage becomes a low level "L" and which low level "L" is in turn fed to a reset terminal RESET of the CPU 1 as a reset signal.
The CPU 1 resets a built-in program counter, register and the like to an initial state automatically upon receiving the reset signal of low level "L".
The capacitor 206 is charged to its full after a certain period of time, passes and in turn the flow of the base current is stopped, causing the transistor 204 to turn off. Hence, the collector voltage of the transistor 204 is brought to a high level "H" from the low level "L", thus releasing the reset of the microcomputer.
As an example of the apparatus having a built-in microcomputer, there is a television receiver equipped with a remote controller. It is necessary for such television receiver to keep an operator command receiving circuit alive in order to receive a power on/off command or other required operator commands even if a main power switch is turned off. Therefore, as it is apparent from the operation of the remote controller, in the television receiver of the type described above, the microcomputer and other peripheral circuits that constitute the remote controller are supplied with power at all times regardless of a switching state, or on/off, of the main power switch for the television receiver for maintaining them in a stand-by condition in order to receive the operator commands.
Accordingly, to reset the built-in microcomputer for initialization in case of an abnormal operating condition of the apparatus such as a program runaway, a maintenance period or other cases as required, pulling the attachment plug from the receptacle disconnect the power supply from the apparatus, and plugging the attachment plug back in the receptacle once again has been the only way to reset the microcomputer. Thus, it has been troublesome to reset the microcomputer heretofore.
It is therefore an object of this invention to eliminate the problems encountered in the prior art and to provide a reset circuit for a built-in microcomputer wherein the microcomputer is reset easily without disconnecting the power supply to an apparatus.